Generally, twin roll strip casting is a process including supplying molten steel to two rolls that are rotating, and continuously producing a strip having a thickness of several mms directly from the molten steel.
FIG. 1 is a perspective view of a typical twin roll strip caster. FIG. 2 is of schematic views showing skull formed in an edge dam according to a conventional technique.
As shown in FIG. 1, in the typical twin roll strip caster, molten steel is uniformly supplied from a tundish into the space between two casting rolls 110 by a nozzle 120, and the casting rolls 110 rotate. Then, molten steel forms solidified layers on the surfaces of the casting rolls 110 that are being cooled, and the solidified layers unite together with each other at the closest point between the casting rolls 110, thus continuously forming a strip having a predetermined thickness.
Two edge dam refractories 150 are respectively provided on opposite ends of the pair of casting rolls 110 to prevent molten steel from flowing out of the space between the casting rolls 110. Both high temperature molten steel that has been supplied between the casting rolls 110 and the casting rolls 110 that are being cooled by water are simultaneously put in contact with the active surfaces of the edge dam refractories 150 that have been preheated before casting. Hence, of the surfaces of the edge dam refractories 150, portions that make contact with the casting rolls 110 cool rapidly, causing heat loss in the vicinity thereof, thereby forming conditions under which molten steel can easily solidify.
Therefore, as shown in FIG. 2, molten steel 131 is solidified on the active surfaces of the edge dam refractories 150, thus forming edge skull 132 and surface skull 134. Such skull grows on the surfaces of the edge dam refractories 150. Of the skull, the edge skull 132 undergoes repeated growth and removal and then becomes mixed with the edges of a casting strip 140, deteriorating the quality of the casting strip 140. In addition, when the skull hardens, it is compressed, forming lower skull 133 between the casting rolls 110, thus causing damage to the casting rolls 110, or inducing the strip to break.
In an effort to overcome the above problems, a technique of injecting an inert gas into the molten steel through the lower portion of the edge dam and preventing the molten steel from solidifying, and a technique of oscillating the edge dam refractories at a predetermined amplitude and physically removing the skull were proposed.
The inert gas injection method of the skull removal techniques is a technique in which a thin metal tube is installed on the lower portion of each edge dam refractory and an inert gas is injected into the molten steel through the metal tube, thus preventing the molten steel from solidifying, and reducing skull. This technique is comparatively effective at reducing skull of the lower portion of the edge dam, but there still remains the problems of the generation and growth of surface skull on the inner surface of the edge dam and of edge skull on junction surfaces between the casting rolls and the active surface of the edge dam.
As shown in FIG. 3, the edge dam oscillation method of the skull removal techniques is a technique which oscillates the edge dam refractories at a predetermined amplitude, thus physically removing skull. In this technique, when an oscillation motor (not shown) is operated, an eccentric shaft 330 rotates. Thereby, a slide bushing 320 comes into contact with a cover 310, thus generating oscillation. The oscillation is transmitted to an oscillation plate 300, so that as shown in FIG. 4a, the lower portion of the oscillation plate oscillates around a bearing 301 provided on the center of the oscillation plate in the same manner as that of a pendulum, thus oscillating the edge dam refractory 142, thereby preventing the skull from fusing with the edge dam.
However, this technique pertains to a mechanical oscillating method using an oscillation cam 302, which is fixed in amplitude. Thus, when it is necessary to change the amplitude, it is required to replace the eccentric shaft 330, an eccentric ring or others with new ones before casting, and depending on a worker, the amplitude may be different. Hence, even if edge skull forms during the casting, it is impossible to control the amplitude, forcing the casting operation to be interrupted. Moreover, because the edge dam refractory oscillates in the same manner as that of a pendulum, the upper and lower portions of the edge dam reliably oscillate, but as it becomes closer to the center of the edge dam, the amplitude reduces, and a dead zone 200 that does not oscillate is eventually formed at the center of the edge dam. In the dead zone, skull is still formed and grown, causing the problem of mixing with a casting strip. Meanwhile, although it is possible to increase the amplitude to prevent the occurrence of the dead zone, this may damage the edge dam, and fragments of the damaged edge dam may mix with a casting strip.